Al,
If temperature distribution is your primary concern, why not just run more
fans They're probably less expensive to operate than the heaters.
The key to efficiency is to match the heating capacity to the greenhouse
demand. The ideal is a heater that puts out the exact amount of energy that
the greenhouse loses, such that it is on constantly. Obviously it's
impossible to have such a device, as outside temperature changes and
sunlight affect the balance.
As there is no way to actually meet that ideal, one tries to size the heater
to be able to keep up with the demand under your worst case scenario - in my
case with an "even worse than that" built in safety factor - knowing that it
will operate less efficiently under "better-than-worst-case" conditions.
If one of your heaters has the capacity to heat your greenhouse, it is more
efficient to heat exclusively with one, and have the second as a backup with
the thermostat for it set lower than the primary. If each your heaters has
half of the necessary capacity, using them both with be immeasurably less
efficient than running a single unit with twice the capacity, the difference
being the amount of oil that has to be burned in order to heat the extra
mass of metal.
--
Ray Barkalow - First Rays Orchids -
www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!
"Al" wrote in message
...
So you all are saying that I am burring more oil by what I am doing:
by running two identical heaters at 63.5 instead of one heater at 65 to
maintain an average of 65 in the space I will spend more money?
"heating double the mass of steel and running each for half the time"
uses more oil?
The extra BTUs required to
"maintain the part of the greenhouse 2.5 degrees above normal would not be
made up with the savings of only heating the other side to 2.5 degrees
below
normal" so I will use more oil?
Lacking some calculus equation I could not understand anyway, I'll take
your word for it. :-) I do kind of see what you are saying. So the
smartest thing I can do (since I was dumb enough to buy these inefficient
heaters anyway) is to set one heater at 65 and one at 63 so the second
unit would only come on if the outside temps/winds stressed the first
unit's ability to maintain the temperature or failed to start at all. My
goal is to use the least amount of oil while knocking the fewest buds off
the phals.
I do like the nice even heat distribution I get with both heaters. I kind
of think some of the bud loss I got last year was from marauding cold dry
drafts in the circulating pattern. I will have to look into a way to
distribute the heat from one unit more evenly. Those clear plastic tubes
that hook in front of some heater brands and run down the length of the
greenhouse dispensing heated air though little holes will not work because
they can not be attached to the front of my heaters. The manufactures
told me so last year. If these things would just hurry up and rust away,
I would invest in a better method of heating; probably a boiler with
fin-tubes of circulating water running under the benches.
"Ray" wrote in message
...
It is less-efficient to run two.
When a heater comes on, you pay to heat the heater in addition to heating
the air. If you plotted a curve of the percentage of energy spent
heating the hardware versus heating the air, it gets more efficient with
longer individual run times. The longer the burn time per start, the
more efficient it is. That's why it doesn't pay to buy a 100K BTU heater
when your need is for a 50K unit - it may heat the air really fast, but
it has to do so more often. The ideal would be a heater that generated
heat continuously at the exact rate the greenhouse lost it.
With two heaters, you are heating double the mass of steel and running
each for half the time for each burn cycle.
--
Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!
"Al" wrote in message
...
ps. logically I come up with the same btu's must be needed to reach the
same average temperature, but is this the truth?
"Al" wrote in message
...
Hi,
I have a 30 by 100 foot greenhouse. I have two heaters, forced air
heaters, that burn oil. One heater alone is enough to heat the
greenhouse (on all but the coldest nights). They hang on opposite side
of the long greenhouse and the un-used one is a back up incase the
primary one dies. Two thermostats hang in the middle of the space, 50
feet from the heaters about midway way from the floor to the roof. One
thermostat controls one heater each and they are independent of each
other. There are four temperature monitoring stations in the
greenhouse that are independent of the on/off thermostats that are
situated to read the two coldest spots and the two warmest spots. The
four spots are gathered from plant level around the bench levels, (not
extremely close to heaters or tucked down in cold corners)
I can run the south wall heater with its thermostat set at 65 (in the
middle of the greenhouse) and the north end of the greenhouse will
register around 63 and the heated side will register around 68 but the
air is circulated by LOTS of fans and the center air mass where the
thermostat is, is 65 degrees. The four temperature sensors around the
greenhouse average 65. The same pattern emerges no matter which heater
I use; the far side is cool and the heater side is hotter but the
middle is 65.
If I set *both* heaters to come on at about 64, both sides of the
greenhouse heat to around 68 and the middle of the air mass turns on
and off both heaters at 64. And they don't seem to cycle on and off as
often, and it seems there is probably more radiant heat available from
two heaters but that may be an illusion because the outside night
temperature varies a lot from night to night and this has to effect how
the heaters cycle on and off.
....anyway the average temperature based on the 4 monitoring stations
now comes to just under 66.... I have to set the thermostats just
about 63.5 to get an average air temperature of 65, so it matches the
average air temperature of the single heater number.
Here's my question, so by running two identical heaters at 63.5 instead
of one heater at 65 am I using more oil, less oil, or the same amount
of oil to heat the same space to the same average temperature? there
has to be an equation that will answer this question. Anybody know how
to figure it?